Permitted transfer distance for current signal 4-20mA

Dear all,

Could anyone know about permitted transfer distance for current signal 4-20mA of Transmitter? I am using pressure transmitter SITRANS P220, the distance from transmitter to PLC module is about 450m, is it possible to connect current signal of transmitter directly to PLC module?

In case of using repeater, which one  should be chosen and how many repeater needed?

Please refer attachment for P220 specification.

There is no one answer.   The answer depends on your specific
- power supply voltage
- receiver's analog input resistance and
- the resistance of the copper wire cable used.

The most commonly used power supply is a 24Vdc power supply, so that's the example used here.

The total resistance that the loop can support can be found by using the formula in the data sheet:  Load = (UB-10V)/0.02A

--Deleted Link, Picture is added as an attachment--

The "Load" is the total loop resistance through which 20.0mA will flow at a given power supply voltage.  The formula is actually Ohms Law  R = E/I or R = V/A

Load = (UB-10V) / 0.02A

UB is the power supply voltage:   24V DC

(UB - 10V) = 24V - 10V = 14V

R = 14V/0.02A = 700 ohms

A 24V DC power supply will drive 20mA through 700 ohms of external resistance (accounting for the 10V drop in the transmitter).

The receiver device is the analog input (AI) that receives the 4-20mA signal, the indicator or controller or PLC or recorder, whatever.  All current signal analog inputs use a dropping or shunt resistor to develop an IR or voltage drop which the electronics 'read' to get the signal.

You need to know what that AI resistance is.   250 ohms is common, but not universal by any means.   I have seen 1, 5, 12.5, 20, 50, 57.5, 100, and 250 ohms resistance values used for analog inputs.   The spec for the analog input states its input resistance.

For example, if the analog input resistance were 250 ohms, then the loop could support 450 ohms of copper wire resistance (700 - 250 = 450).

Here's some North American values (using AWG sizing) for copper wire resistance.
AWG = Ohms / 1,000 feet
22 = 19.0
20 = 11.9
18 = 7.51
16 = 4.73
14 = 2.97
12 = 1.87

20G conductors at about 12 ohms per 1000 feet (about 330m) means that 450 ohms of 20G conductor could be support 4-20mA as far as about 18,750 feet, about 3.5 miles or about 6km.


 

Dear Danw,

Your calculation method and explanation are wonderful, it's very helpful. As any cases, we need the Load of transmitter to specify the transfer distance.

Thank you so much for your help. 

Hi,

Good explanation! How did you calculate the distance for 450ohms?

Hi Thai,

don't forget you need to go there and back...